Interpretive Summary: Managing phosphorus loss from agriculture requires an understanding of how landscape processes interact with field management to influence phosphorus transfers. We investigated the dynamics of phosphorus transfers in runoff from an agricultural landscape in central Pennsylvania to connect hydrologic variability by landscape position with the potential for phosphorus loss in runoff. Phosphorus transfers were dynamic in time and space, but occurred primarily in areas where soil conditions favored the largest amounts of runoff. Findings support the current model for targeting remedial practices to small areas of the landscape responsible for the majority of phosphorus loss in runoff.

Technical Abstract:
Phosphorus (P) losses from agricultural landscapes arise from the interaction of hydrologic, edaphic and management factors, complicated by their spatial and temporal variability. We monitored sites along two agricultural hillslopes to assess the effects of field management and hydrology on P transfers in surface runoff at different landscape positions. Surface runoff varied by landscape position, with saturation excess runoff accounting for 19 times the volume of infiltration excess runoff at the north footslope position, but infiltration excess runoff dominating at upslope landscape positions. Runoff differed significantly between south and north footslopes, coinciding with the extent of upslope soil underlain by fragipan. Phosphorus in runoff was predominantly in dissolved reactive form (70%), with the highest concentrations (mg L-1) associated with upper landscape positions closest to fields serving as major sources of P. These high runoff P concentrations were primarily rapid incidental transfers of P from recently applied fertilizer and manure. However, the largest loads (g) of P were from the north footslope, where runoff volumes were 24 times larger than from all other sites combined. Loads of P from the north footslope appeared to be primarily chronic transfers of desorbed soil P. While runoff from the footslope likely contributed directly to stream flow, hence stream water quality, 27% of runoff P from the upslope sites did not connect directly with stream flow. Findings of this study will be useful for evaluating the critical source area concept and metrics such as the P-Index.